The present invention relates to the polymerization of olefins in a single reactor using bimetallic catalysts, and more particularly to the control of the flow index and/or amounts of polymer composition components (xe2x80x9csplitxe2x80x9d) by the addition of a control agent that, in certain embodiments, is selective for one catalyst component of the bimetallic catalyst composition.
Broad or bimodal molecular weight distribution polymer compositions are compositions that typically include one or more high molecular weight polymers and one or more low molecular weight polymers. In bimodal molecular weight distribution polymer compositions, the weight fraction of the high molecular weight (xe2x80x9cHMWxe2x80x9d) polymer typically ranges from, for example, 0.10 to 0.90 for applications requiring broad molecular weight distribution polymers. The relative amount of HMW polymer in the polymer composition can influence the rheological properties of the composition. One such measurable rheological property of bimodal polymer compositions is its flow index (xe2x80x9cFIxe2x80x9d, or I21, measured at 190xc2x0 C., 21.6 kg according to ASTM D-1238). The I21 of the bimodal polymer composition, in one embodiment, possesses an I21 that is between 2 and 100 dg/min. This range represents a balance between processability (desiring relatively high I21) on the one hand, and product (film, etc.) toughness (desiring relatively low I21) on the other hand. Hence, it is necessary to control polymer composition I21 in the polymerization reactor.
One desirable method of producing bimodal polymer compositions is through the use of a bimetallic catalyst composition in a single reactor such as disclosed in, for example, U.S. Pat. No. 5,882,750 and U.S. Pat. No. 6,274,684. Such compositions incorporate at least two, preferably two, metal centers, both of which may be the same or different metal with similar or differing coordination spheres, patterns of substitution at the metal center or ligands bound to the metal center. Typically, one of the metal centers produces a low molecular weight (xe2x80x9cLMWxe2x80x9d) polymer while the other produces a HMW polymer in the single polymerization reactor, and desirably, although not necessarily, simultaneously. U.S. Pat. No. 5,525,678 to Mink et al. discloses the use of water and carbon dioxide in controlling the I21 using one such bimetallic catalyst composition. WO 02/46246 to Mawson et al.; U.S. Pat. Nos. 6,420,474 and 6,569,963 to Nowlin et al. disclose the addition of an additional catalyst to adjust the relative amounts of HMW and LMW polymers in a polymer composition. However, the products resulting therefrom, such as bimodal resins used to make films, may still suffer from gel formation, the reaction process itself may be subject to fouling, which causes an undesirable need to shut down the polymerization reactor, and further, the procedure of adding a catalyst component can add cost and complexity to the process.
What is needed is an improved method of controlling I21 in the manufacture of bimodal polymer compositions when using bimetallic catalyst compositions. While many types of compounds have been disclosed for use in polymerization reactors, for example in WO 01/00691; U.S. Pat. No. 6,596,824; U.S. Pat. No. 6,417,299; U.S. Pat. No. 6,417,298; U.S. Pat. No. 6,313,236; U.S. Pat. No. 5,733,988, U.S. Pat. No. 5,625,012; U.S. Pat. No. 5,244,987; U.S. Pat. No. 5,200,502; U.S. Pat. No. 5,194,526; U.S. Pat. No. 5,066,736; U.S. Pat. No. 4,855,370; and EP 1 159 311, EP 0 630 910 and EP 0 315 192; and Soviet Union Patent No. 1 249 025, and specifically, the use of aluminum compounds, for example RE 33,683, U.S. Pat. No. 6,420,474 and U.S. Pat. No. 6,569,963, and EP 0 315 192; there is still the problem of effective I21 control in the manufacture of bimodal polymer compositions when using bimetallic catalyst compositions. The inventors have unexpectedly found that there exists a broad class of compounds useful as I21 control agents, or xe2x80x9csplitxe2x80x9d control agents, that also have other advantages as described herein.
The present invention is directed to methods of controlling the rheological properties of a polymer composition generated by a bimetallic catalyst system in a single reactor, the control affected by the use of control agents; the control agents are added in an amount sufficient to alter the relative ratios, or xe2x80x9csplitxe2x80x9d of the HMW and LMW polymer components of the polymer compositions. More particularly, the control agents are used as described herein to counter the rheological-altering influences in bimetallic catalyst systems of such compounds as alkanes and aluminum alkyls.
One aspect of the present invention is a method of producing a polymer composition in the presence of rheological-altering compounds comprising incorporating a high molecular weight polymer into a low molecular weight polymer to form the polymer composition in a single polymerization reactor in the presence of polymerizable monomers, a bimetallic catalyst composition and at least one control agent; wherein the control agent is added in an amount sufficient to control the level of incorporation of the high molecular weight polymer, the level of low molecular weight polymer, or both.